1. Field of the Invention
The present invention relates to a controller for an injection molding machine capable of achieving precision metering of resin.
2. Description of the Related Art
In an in-line screw injection molding machine, a screw in a heating cylinder is rotated to melt and knead a resin material, and the molten resin is fed to the tip portion of the heating cylinder under pressure. Then, the screw is retreated under pressure control. When the screw reaches a set metering point, its rotation and retreat are stopped, and metering of the resin is performed. After this metering, the screw is advanced to inject and fill the molten resin into a mold.
Dispersion of the fill of the molten resin in the mold should be reduced in order to improve the quality of molded products. The molten resin is injected after the screw is retreated to the position of the metering point. If the metered molten resin is directly injected and filled into the mold when this is done, an accurate fill can be obtained, so that the molded products are not subject to dispersion in weight and their quality can be kept uniform. There is a problem, however, that the molten resin flows backward to change the amount of injected resin as an injection process is started on completion of the metering process, so that an accurate amount of molten resin cannot be injected.
A valve, such as a check valve or check ring, is attached to the tip of the screw, whereby the molten resin produced by the rotation of the screw can be fed to the tip portion of the cylinder in a metering process, and the molten resin at the tip portion of the screw can be injected into the mold by virtue of not flowing backward in the injection process. This valve prevents the molten resin from flowing backward and changing the metered resin amount during the time period between the completion of metering and the start of injection. Thus, the metered amount of resin can be injected accurately. However, there is a problem that the molten resin may be caused to flow backward by delayed operation of backflow preventing means, such as a valve for preventing backflow of the molten resin, so that an accurate amount of molten resin cannot be injected. Various techniques have been proposed to improve this problem.
According to a technique disclosed in Japanese Patent Application Laid-Open No. 60-76321, for example, after a molten resin is plasticized and metered, pilot injection is performed such that a pressure lower than the pressure of injection of the molten resin into the cavity of the pressure chamber of an injection cylinder is maintained for a fixed period of time or so that it increases to a fixed level. During the pilot injection, moreover, a screw is rotated in the direction opposite to the rotation for the plasticization and metering. Thus, a pressure difference is produced between the front and rear sides of a check valve to retreat the valve.
According to a technique based on a post-metering process disclosed in Japanese Patent Application Laid-Open No. 2006-327127, a screw is allowed to freely rotate as it is advanced after a main metering process is finished, and the rotating state of the screw during the advance is monitored. When the screw is stopped from rotating, in this case, it is reversely rotated by a predetermined amount.
According to a technique disclosed in Japanese Patent Application Laid-Open No. 2007-253388, a screw is stopped from rotating when a set metering completion position is reached, and an injection servomotor is driven to advance the screw. By doing this, a pressure P2 higher than a back-pressure P1 is applied to close a check valve. Thereafter, a servomotor for screw rotation is driven to reversely rotate the screw by a fixed amount under the pressure P2, and a pressure p1 of a molten resin in a cylinder is reduced to zero or p2 as close to zero as possible.
According to a technique disclosed in Japanese Patent Application Laid-Open No. 2007-253606, a screw or plunger having undergone metering is advanced forward at a predetermined speed in an injection direction for a predetermined time or so that a predetermined pressure or position is reached, after a retreat process in which the screw is retreated in the direction opposite to the injection direction. Thereafter, the screw is rotated at a predetermined speed in the direction opposite to that in a metering process.
In any of the prior art techniques described above, the screw is advanced to increase the resin pressure in order to close the check valve after the end of metering, and is reversely rotated at the same time or thereafter. In this case, the resin pressure is increased to a somewhat high level during the screw advance, whereby a pressure difference is produced such that the pressure (resin pressure) of the check valve on the screw-tip side is higher than the pressure on the screw-root side. This pressure difference produces a force to move the check valve from the tip side of the screw to the root side, thereby closing the check valve. In order to close the check valve reliably and steadily during the screw advance, therefore, the resin pressure should be increased to a certain level or higher (see FIG. 8). Depending on the type of the mold or molded products, however, application of a high pressure after the end of metering may cause problems on molding. The foregoing prior art techniques cannot be used in such a case.